Method to achieve diversity in a communication network

ABSTRACT

A method of wirelessly communicating between units comprises using at least two repeaters between a transmit unit and a receive unit. The repeaters enable a diversity scheme to be emulated.

BACKGROUND OF THE INVENTION

[0001] The present invention is related to a method of creating spatialdiversity in wireless communication networks.

[0002] Wireless communication systems have exploded in popularity inrecent years. Cellular phones, pagers, and more recently Bluetoothdevices all take advantage of these wireless communication systems. Acommon obstacle to wireless communications is a phenomenon known asfading, where messages transmitted between two units in the system arelost or garbled.

[0003] One technique to combat fading is using diversity schemes such asa spatial transmit diversity scheme. Transmit diversity schemes send atleast two signals from distinct antennas or antenna elements. Thesignals contain parts or all of the same message. The goal of transmitdiversity it to alleviate the effects of fading. The signals aretransmitted in such a way that they do not cancel each other at thereceiver. The receiver can process the signals to exploit diversity andimprove performance. Transmit diversity can be viewed as a dual oftraditional receive diversity, where signals from multiple receiveantennas are processed by the receiver. Transmit and receive diversitycan also be used together to obtain further improvements. Transmitdiversity methods include delay diversity, the Alamouti code, theLindskog-Paulraj technique, space-time codes, BLAST, and other similarmethods. Some transmit diversity schemes require multiple transmitantennas, which may be difficult to implement in small, inexpensivewireless devices. For example, Bluetooth devices typically have a singletransmit antenna operative in the Bluetooth frequency band.

BRIEF SUMMARY OF THE INVENTION

[0004] The present invention may be used in ad-hoc networks such as arecreated in Bluetooth systems. In particular, a transmit unit, using asingle transmit antenna may send a message to at least two intermediateunits that act as repeaters to send the message to a receive unit. Byrouting the message through the two repeaters, spatial transmitdiversity is created.

[0005] To route the message through the two repeaters, the message maybe encoded into two distinct signals, each of which is sent to adifferent repeater. The encoding process may use any number of codingschemes such as an Alamouti code, a Linskogg-Paulraj code, or the like.

[0006] Further, the present method of using repeaters may be used tofine tune power control feedback loops. The receiver may control therepeaters and the repeaters may control the originating station suchthat power is conserved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a block diagram of a wireless communication systemaccording to one embodiment of the present invention;

[0008]FIG. 2 is a flow diagram illustrating one embodiment of a powercontrol method associated with the present invention;

[0009]FIG. 3 is a flow diagram illustrating a second embodiment of apower control method associated with the present invention; and

[0010]FIG. 4 is a functional block diagram of a mobile terminal such asmay be used in conjunction with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention relates to diversify techniques forwireless communication network. In particular, the present inventionprovides a technique of creating special diversity between a transmitunit using a single antenna and a receive unit using a single antenna.Other forms of diversity may also be created. Spatial diversity, as usedherein, is defined to be those situations where diversity is achieved byantenna separation. This is conventionally achieved by using a pluralityof antennas at either the transmitter or receiver. According to thepresent invention, partial diversity is achieved by using a plurality ofrepeaters, interposed between a transmitter and a receiver. Thisinvention is primarily directed at ad hoc networks, such as a Bluetoothnetwork, but is also applicable to WCDMA-TDD systems and other types ofnetworks.

[0012]FIG. 1 illustrates the basic concept of the present invention. Awireless communication network 10 comprises a transmit unit 20, a firstintermediate unit 30, a second intermediate unit 40, and a receive unit50. An obstacle 60 may be present within the network 10. Obstaclesinclude geographic or man-made features that inhibit wirelesscommunication as well as environmental factors that inhibit wirelesscommunication. While obstacles 60 may encourage the use of the presentinvention, they need not be present to justify using the presentmethodology. Transmit unit 20 uses a single transmit antenna 22 withinan operative frequency band. Likewise, receive unit 50 uses a singlereceive antenna 52 within the operative frequency band.

[0013] In one embodiment, the network 10 is an ad hoc network, such asenvisioned by the Bluetooth standard. The Bluetooth standard enablesseamless communication of data and voice over short-range wireless linksbetween both mobile devices and fixed devices. The Bluetooth standardpermits ad hoc networking of devices equipped with a Bluetoothinterface. Different Bluetooth devices can automatically connect andlink up with one another when they come into range to form an ad hocnetwork, generally referred to as a piconet. The Bluetooth standardspecifies how mobile devices, such as phones, personal digitalassistants (PDAs), and wireless information devices (WIDS), caninterconnect with one another and with stationary devices, such asdesktop computers, printers, scanners, and stationary phones.

[0014] Bluetooth devices operate in the Industrial-Scientific-Medical(ISM) frequency band at approximately 2.45 GHz. The ISM band is anunlicensed frequency band. The Bluetooth standard employs spreadspectrum techniques that provide a high degree of interference immunityand multiple access. In particular, the Bluetooth standard employs aspread spectrum technique called frequency hopping to spread anarrowband signal over a wide spectrum of frequencies. This techniquespreads a narrowband signal by “hopping” from one frequency to anotherin a defined sequence in accordance with a pseudo-random code and at adefined hop rate.

[0015] Frequency hop systems divide the frequency band into a pluralityof hop carriers or frequencies. Each hop channel comprises a definedsequence of frequency hops. The Bluetooth standard defines seventy-ninehop frequencies with one MHz spacing. A hop channel comprises aparticular sequence of frequency hops. A hop channel is divided into 625microsecond intervals—called slots—each corresponding to a different hopfrequency. Thus, the Bluetooth device hops from one hop frequency toanother, remaining on each hop frequency for a period of 625microseconds, giving a nominal hop rate of 1,600 hops per second. Onepacket can be transmitted per slot or hop. Slots within a hop channelare alternately used for transmitting and receiving, which results in atime division duplex (TDD) scheme.

[0016] Each hop channel is determined by the hop sequence (the order inwhich the hop frequencies are visited) and by the phase of the hopsequence. Two or more units sharing the same hop channel form a piconet,where one unit acts as a master—controlling traffic on the piconet—andthe remaining units act as slaves. Under the Bluetooth standard, the hopsequence is determined by the master unit's system clock. The slaves usethe master identity to select the same hop sequence and add time offsetsto their respective native clocks to synchronize to the master unit.

[0017] The last bit of the Bluetooth puzzle that may be helpful inunderstanding the basics of the standard is how the piconets are formed.Every Bluetooth device is identified by a unique address called theBluetooth device address. A first device obtains this address from asecond device through a procedure called an “inquiry.” When the firstdevice invokes the inquiry procedure, all listening devices in range ofthe first device will respond to this inquiry by returning a responsethat includes inter alia the Bluetooth device address of the respondingdevice. The standard has provisions for preventing the respondingdevices from responding all at the same time. Thus, after the inquiryprocedure, the first device has the Bluetooth device address for allBluetooth devices within range of the first device. The first device maynow establish a connection to form a piconet. The procedure forestablishing this connection is called “paging.” A page is alwaysdirected towards one device, typically, one of the devices for which aresponse to an inquiry was received. However, the paging device mayalready have the Bluetooth device address of the paged device withoutthe need for sending an Inquiry message to obtain the address. When thefirst device initiates a page to the second device, the second deviceanswers the page and synchronizes itself to the first device's hopchannel, while also offsetting its internal clock. Thus, for thatpiconet, the paged device becomes a slave unit with respect to thepaging device. There may be an optional authentication step in thisprocess if needed or desired.

[0018] In network 10, any of the units 20, 30, 40, or 50 may be themaster unit. However, for the purpose of further description, it isassumed that the transmit unit 20 is the master and connects to thereceive unit 50. While establishing that connection, the transmit unit20 and the receive unit 50 may negotiate a protocol to specify howfurther communications between the two will be conducted. Thisnegotiation may cover whether diversity is to be employed and who willbe the master. From this initial negotiation, the two units 20, 50 mayrecruit the intermediate units 30, 40 to serve as repeaters as describedbelow. This negotiation step may involve an extension or modification ofthe existing Bluetooth standard to implement the present invention. Iftransmit unit 20 and receive unit 50 elect to implement diversity infurther communications, intermediate units 30 and 40 are notified,typically by the transmit unit 20to serve as repeaters. Intermediateunits 30 and 40 relay messages from transmit unit 20 to receive unit 50,as hereinafter described, in a fashion that creates a spatial diversitydespite the fact that the transmit unit 20 and the receive unit 50 onlyuse a single antenna 22, 52 respectively, at the operative frequencyband to transmit and receive the message.

[0019] In one exemplary embodiment of the invention, transmit unit 20processes a message to be transmitted to produce two distinct signals s₁and s₂. A message, as used herein, comprises the information that thetransmit unit 20 wishes to convey to the receive unit 50. It maycomprise voice, data, or some other information as needed or desired.Transmit unit 20 may use an Alamouti coder to process the message intothe two distinct signals. Other codes are also contemplated, such as theLinskogg-Paulraj code. Transmit unit 20 sends signal s₁ to a firstintermediate unit 30 and signal s₂ to a second intermediate unit 40. Thetwo signals may be transmitted simultaneously on different frequencies,or sequentially on the same frequency. In a CDMA system, such asWCDMA-TDD, the two signals can be transmitted simultaneously ondifferent spreading codes, which provide orthogonality orquasi-orthogonality, enabling the receiver to distinguish between them.More generally, each of the two signals can be transmitted on a numberof spreading codes. In either case, intermediate units 30, 40 retransmittheir respective received signals s₁ and s₂ on the same frequency and atthe same time to the receive unit 50. The signal r received at receiveunit 50 can thus be modeled as:

r=a ₁ s ₁ +a ₂ s ₂ +n  Eq. (1)

[0020] where a₁ and a₂ represent the cumulative fading channels betweentransmit unit 20 and receive unit 50 through intermediate units 30, 40,respectively, and n represents the cumulative noise and interferencefrom transmit unit 20 to receive unit 50. Receive unit 50 processes thereceived signal r to recover the original message. Such processing iswell known in the art. To summarize briefly, receive unit 50 generateschannel estimates of a₁ and a₂ and then uses these channel estimates togenerate estimates of signals s₁ and s₂. The estimates of s₁ and s₂ arethen provided to a decoder, which process the estimates of s₁ and s₂ togenerate an estimate of the original message. For example, if Alamoutiencoding is used, receive unit 50 would include an Alamouti decoder tocombine or decode signals s₁ and s₂. Of course, other diversitycombining techniques could be employed by receive unit 50.

[0021] Equation (1) assumes flat fading channels with a single tap, butthe extension to multi-tap channels is well within the skill of thoseproficient in the art. If intermediate units 30 and 40 are separatedenough in space, then the cumulative fades are uncorrelated. In thiscase, equation (1) corresponds with the received signal in a system withtwo antenna transmit diversity. To this extent, the network 10 hascreated a spatial transmit diversity scheme. The present invention maybe particularly useful in a Bluetooth network, spatial diversity is ahelpful feature, as most units 20, 30, 40, and 50, will only use asingle antenna at the ISM frequencies.

[0022] This concept is easily extended. For example, intermediate units30, 40 may retransmit their received signals to receive unit 50 twice.In one variation, the retransmitted signals s₁, s₂ may be sent at afirst time on a first common frequency and at a second time on adifferent common frequency. In a second variation, the retransmittedsignals s₁, s₂ may be transmitted simultaneously at a first time, andagain at a second time. The signals r₁ and r₂ received at receive unit50 may be expressed as:

r ₁ =a ₁ s ₁ +a ₂ s ₂ +n ₁  Eq. (2)

[0023] and

r ₂ =a ₁ ′s ₁ +a ₂ ′s ₂ +n ₂  Eq. (3)

[0024] Equations (2) and (3) correspond with the received signals for asystem with two transmit antennas and two receive antennas. Receive unit50 processes received signals r₁ and r₂ as previously described torecover the original message. Thus, not only is transmit diversitycreated, but also receive diversity is created.

[0025] In another variation of the invention, intermediate units 30, 40may retransmit their respective received signals s₁, s₂ on differentfrequencies or at different times to the receive unit 50. The signals r₁and r₂ received at receive unit 50 may be expressed as:

r ₁ =a ₁ s ₁ +n ₁  Eq. (4)

[0026] and

r ₂ =a ₂ s ₂ +n ₂  Eq. (5)

[0027] This variation may simplify processing of the message at thereceive unit 50 as there is no crosstalk between the signals s₁ and s₂that needs to be separated. It should be noted that even in networks 10that require intermediate units 30, 40 to retransmit their respectivereceived signals s₁, s₂ on adjacent frequencies and/or times, spatialseparation of intermediate units 30, 40 still provides some form ofdiversity.

[0028] In all the variations, it is expected that the transmit unit 20instructs the intermediate units 30, 40 on when and how to retransmitthe signals, s₁, s₂ but it is possible, especially when the receive unit50 is the master unit, that the receive unit 50 is providing theseinstructions.

[0029] Units 20, 30, 40, and 50 may further include power controlfeedback loops therebetween. In many systems there is a mechanism forfast feedback, allowing a transmitting unit (20, 30, or 40) to receiveinformation from the receiving unit (30, 40, or 50) on a reverse channelregarding the quality of the received signal. This signal quality isreflected in large part by estimates of a₁ and a₂ and the noise andinterference level, where a₁ and a₂ represent the fading of the channelsas previously defined. The feedback is useful to the transmitting unit20, which can adjust its transmit power accordingly, using one ofseveral known strategies.

[0030] Exemplary flow charts illustrating this power control method arepresented in FIGS. 2 and 3. In FIG. 2, the transmit unit 20 transmitssignals s₁ and s₂ to the intermediate units 30, 40 (block 100).Intermediate units 30, 40 evaluate the respective received signals s₁and s₂ (block 102). This evaluation may include an estimation of fadingon the channels that exist between the transmit unit 20 and theintermediate units 30, 40. Intermediate units 30, 40 may also make anestimate of the fading for the complete channels a₁ and a₂ based oninformation received from receive unit 50 on the reverse channels.Intermediate units 30, 40 may send information, possibly includingchannel estimates, as well as other well known parameters, to thetransmit unit 20 on an appropriate reverse channel (block 104). Transmitunit 20 compensates for fading in the propagation channel if needed(block 106). Compensation may be according to any known scheme, such asboosting transmitted power to equalize received power or reducing powerto faded channels and boosting power to unfaded channels. Othertechniques may also be used.

[0031] A similar feedback loop exists between the intermediate units 30,40 and the receive unit 50. This is illustrated in FIG. 3. Intermediateunits 30, 40 transmit their respective signals s₁ and s₂ to the receiveunit 50 (block 110). Receive unit 50 evaluates the received signals(block 112). Again this evaluation may include estimating fading on thechannels. Receive unit 50 then sends information about the receivedsignals to the intermediate units 30, 40 on an appropriate reversechannel (block 114). Intermediate channels 30, 40 may compensate forfading if needed (block 116). This two-stage power control abilityenables the network 10 to do a finer form of power control thancontemplated in other systems.

[0032] As another variation on the present invention, it is possiblethat the intermediate units 30, 40 are the units that separate a messageinto s₁ and s₂. This would require the space-time encoder to be presentat the intermediate units 30, 40, but such is within the scope of thepresent invention.

[0033] Note that the present invention may function with more than tworepeaters in a single stage or multiple stages of repeaters. This allowsthe network 10 to extend the range of the connection or to circumventobstacles 60 as needed or desired. Other reasons for implementing thepresent invention in a network 10 are also contemplated.

[0034] Further note that the present invention is not limited to ad hocnetworks 10, but also may be used in networks with fixed elements, forexample, fixed intermediate units 30, 40. In essence, this method willwork in almost any TDD system.

[0035] The present invention, as previously discussed, is particularlywell suited for use in Bluetooth networks. While Bluetooth networks mayinclude many different types of devices, a typical device for aBluetooth network is a mobile terminal. An exemplary mobile terminal 200is shown in FIG. 4. Mobile terminal 200 comprises a main control unit220 for controlling the operation of the mobile terminal 200 and memory240 for storing control programs and data used by the mobile terminal200 during operation. Memory 240 may be contained in a removable smartcard if desired. Input/output circuits 260 interface the control unit220 with a keypad 280, display 300, audio processing circuits 320,receiver 380, and transmitter 400. The keypad 280 allows the operator todial numbers, enter commands, and select options. The display 300 allowsthe operator to see dialed digits, stored information, and call statusinformation. The audio processing circuits 320 provide basic analogaudio outputs to a speaker 340 and accept analog audio inputs from amicrophone 360. The receiver 380 and transmitter 400 receive andtransmit signals using shared antenna 440. The mobile terminal 200further includes a Bluetooth module 410 operating as previouslydescribed and having a single antenna 412 operating in the ISM band.

[0036] It should be noted that, as used herein, the term “mobileterminal” 200 may include a cellular radiotelephone with or without amulti-line display; a Personal Communications System (PCS) terminal thatmay combine a cellular radiotelephone with data processing, facsimileand data communications capabilities; a Personal Digital Assistant (PDA)may include a radiotelephone, pager, Internet/intranet access, Webbrowser, organizer, calendar and/or a global positioning system (GPS)receiver; and a conventional laptop and/or palmtop receiver or otherappliance that includes a radiotelephone transceiver. Mobile terminals200 may also be referred to as “pervasive computing” devices.

[0037] The present invention may, of course, be carried out in otherspecific ways than those herein set forth without departing from thescope and the essential characteristics of the invention. The presentembodiments are therefore to be construed in all aspects as illustrativeand not restrictive and all changes coming within the meaning andequivalency range of the appended claims are intended to be embracedtherein.

What is claimed is:
 1. A method of communicating a message between twostations, comprising: receiving said message transmitted from a firststation at at least two repeaters, said message transmitted over awireless communication channel; and retransmitting said message fromsaid at least two repeaters to a second station such that spatialdiversity is created between said first station and said second station.2. The method of claim 1 further comprising forming a network comprisingat least said first station, said second station, and said at least tworepeaters.
 3. The method of claim 2 wherein forming a network comprisingat least said first station, said second station, and said at least tworepeaters comprises forming an ad hoc network.
 4. The method of claim 2wherein forming a network comprising at least said first station, saidsecond station, and said at least two repeaters comprises forming anetwork with at least two fixed repeaters.
 5. The method of claim 1further comprising transmitting said message from said first station tosaid at least two repeaters.
 6. The method of claim 5 whereintransmitting said message from said first station to said at least tworepeaters comprises encoding said message into at least two distinctsignals.
 7. The method of claim 6 further comprising transmitting afirst one of said at least two distinct signals to a first one of saidat least two repeaters.
 8. The method of claim 7 further comprisingtransmitting a second one of said at least two distinct signals to asecond one of said at least two repeaters.
 9. The method of claim 1further comprising receiving said message at said second station fromsaid at least two repeaters.
 10. The method of claim 9 wherein receivingsaid message at said second station from said at least two repeaterscomprises receiving distinct signals on different frequencies.
 11. Themethod of claim 9 wherein receiving said message at said second stationfrom said at least two repeaters comprises receiving distinct signals atdifferent times.
 12. The method of claim 1 wherein retransmitting saidmessage from said at least two repeaters to a second station such thatspatial diversity is created between said first station and said secondstation comprises retransmitting the message such that frequencytransmit diversity is created.
 13. The method of claim 1 whereinretransmitting said message from said at least two repeaters to a secondstation such that spatial diversity is created between said firststation and said second station comprises retransmitting the signal suchthat time transmit diversity is created.
 14. A method of communicating,comprising: transmitting a message from a first station to at least tworepeaters; receiving said message at said at least two repeaters;retransmitting said message, via at least two distinct signals, one ofsaid at least two distinct signals transmitted from each of said atleast two repeaters to a second station; receiving said at least twodistinct signals at said second station; combining said at least twodistinct signals to reconstruct said message.
 15. The method of claim 14wherein transmitting a message from a first station to at least tworepeaters comprises transmitting a message from a first station using asingle antenna.
 16. The method of claim 15 wherein transmitting amessage from a first station using a single antenna comprises repeatingtransmission of said message to each of the at least two repeaters. 17.The method of claim 14 wherein transmitting a message from a firststation to at least two repeaters comprises simultaneously transmittinga signal on two different frequency channels.
 18. The method of claim 14further comprising forming a network comprising said first station, saidsecond station, and said at least two repeaters.
 19. The method of claim18 wherein forming a network comprising said first station, said secondstation, and said at least two repeaters comprises forming an ad hocnetwork.
 20. The method of claim 18 wherein comprising said firststation, said second station, and said at least two repeaters comprisesforming a network with at least two fixed repeaters.
 21. The method ofclaim 14 wherein receiving said at least two distinct signals at saidsecond station comprises receiving, using a single antenna, each of saidat least two distinct signals at said second station.
 22. A transmittercomprising: a controller adapted to encode a message into at least twodistinct signals; and a transmit antenna to transmit a first one of saidat least two distinct signals to a first one of said at least tworepeaters and a second one of said at least two distinct signals to asecond one of said at least two repeaters.
 23. The transmitter of claim22 wherein said transmit antenna comprises the sole antenna used at anoperative frequency band.
 24. The transmitter of claim 22 wherein saidcontroller is adapted to transmit identical signals to the at least atwo repeaters sequentially.
 25. The transmitter of claim 22 wherein saidcontroller is adapted to transmit said at least two distinct signals tothe at least two repeaters simultaneously at different frequencies. 26.A communication system, comprising: a transmitter comprising a firsttransmit antenna operative in a first frequency band and operative totransmit a message; at least two repeaters operating in said firstfrequency band, said at least two repeaters operative to receive saidmessage from said transmitter and to retransmit said message; a receivercomprising a first receive antenna operating in said first frequencyband to receive said retransmitted message from each of said at leasttwo repeaters.
 27. The communication system of claim 26 wherein saidtransmitter transmits to said at two repeaters using just said firsttransmit antenna.
 28. The communication system of claim 27 wherein saidtransmitter transmits a signal containing said message to a first one ofsaid at least two repeaters and then transmits said signal containingsaid message to a second one of said at least two repeaters.
 29. Thecommunication system of claim 26 wherein said transmitter transmits saidmessage to said at two repeaters simultaneously on two differentfrequency channels.
 30. The communication system of claim 26 whereineach of said at least two repeaters is spaced from one another.
 31. Thecommunication system of claim 26 wherein said system is formed on an adhoc basis.
 32. The communication system of claim 26 wherein saidrepeaters are fixed repeaters.
 33. An ad-hoc wireless communicationsystem, comprising: a transmit unit using a single transmit antenna; aplurality of intermediate units; a receive unit using a single receiveantenna; and said intermediate units serving as repeaters to pass amessage between said transmit unit and said receive unit therebycreating an effective transmit diversity wireless communication system.34. The wireless communication system of claim 33 wherein said transmitunit processes said message into at least two signals, each of which issent to different ones of said plurality of intermediate units.
 35. Thewireless communication system of claim 34 wherein said transmit unittransmits said at least two signals simultaneously on differentfrequencies.
 36. The wireless communication system of claim 34 whereinsaid transmit unit transmits said at least two signals sequentially on asingle frequency channel.
 37. The wireless communication system of claim34 wherein respective ones of said intermediate units retransmitrespective ones of said at least two signals to said receive unit. 38.The wireless communication system of claim 37 wherein said intermediateunits retransmit said at least two signals on a common channel at thesame time.
 39. The wireless communication system of claim 37 whereinsaid intermediate units retransmit said at least two signals twice atdifferent times.
 40. The wireless communication system of claim 39wherein said intermediate units retransmit said at least two signalssimultaneously on a first common frequency channel and again on a secondcommon channel.
 41. The wireless communication system of claim 39wherein said intermediate units retransmit said at least two signalssequentially on a first common frequency channel at a first time andagain on said first common frequency channel at a second time.
 42. Thewireless communication system of claim 37 wherein said intermediateunits retransmit said at least two signals on different frequencies. 43.The wireless communication system of claim 37 wherein said intermediateunits retransmit said at least two signals at different times.
 44. Thewireless communication system of claim 33 further comprising a powercontrol feedback loop.
 45. The wireless communication system of claim 44wherein said power control feedback loop exists between said transmitunit and said plurality of intermediate units.
 46. The wirelesscommunication system of claim 44 wherein said power control feedbackloop exists between said plurality of intermediate units and saidreceive unit.